AGGREGATION OF CHLOROPHYLL a SPECIES ABSORBING NEAR 700nm–1. THE INFRARED CARBONYL BANDS

Amorphous chlorophyll a obtained by electrodeposition(Chl–678) undergoes a bathoch‐romic shift upon solvation with gaseous alcohol at ambient temperatures. The process is reversible when the alcohol is removed. With 1‐propanol, the red band of Chi a is displaced from 678 nm to 701 nm and a new aggregate is formed,Chl–701. To determine the nature of the new aggregate, the IR carbonyl bands of Chi are monitored. The free ketone massive band ofChl–678, situated at 1691 cm ‐1 , is decreased upon solvation to the profit of the associated ketone carbonyl massive band which absorbs at 1656 cm ‐1 . The bandwidths of the massive bands are much greater than the typical value of a carbonyl band of a ketone in solution. This is an indication that the massive bands are composed of overlapping bands. In order to determine the number and the position of the bands, the Fourier self‐deconvolution technique has been used to enhance the spectral characteristics of the infrared spectra. The band fitting technique is then used on the original spectra to determine the difference in intensity between the bands ofChl–678 and those ofChl–701. The interpretation of the resolved IR spectra shows that the amorphous sample is composed of open dimers linked together by the CO‐Mg coordinate bonds. When the solvated species are formed, these coordinate bonds are not ruptured and one molecule of alcohol for two molecules of chlorophyll is linked to these molecules by an hydrogen bond between the hydrogen of the alcohol and the available ketone and by a coordinate bond between the available Mg and the oxygen of the alcohol. Closed dimers are thus formed.